1. Field of the Invention
The present invention relates to an image capturing apparatus that converts light into electrical signals, and to an image capturing system that comprises the image capturing apparatus.
2. Description of the Related Art
Conventionally, image capturing systems have set gains close to the saturation value in order to improve the S/N ratio. However, due to the characteristic fluctuations of sensors, the predetermined signal range could not be attained even with the gain adjustment conducted in the factory. Further, there were cases in which attempting to attain the predetermined signal range would result in a deteriorating S/N ratio. Here, N (noise) refers to the circuit noise existing outside of the sensor.
Japanese Patent Application Laid-Open No. 2003-101881 discloses a method to enlarge the dynamic range of light detection by applying different gains to the ordinary signal and to the excessive signal respectively.
FIG. 8 is a block diagram of the image capturing system disclosed in Japanese Patent Application Laid-Open No. 2003-101881. Many pixels are positioned in a matrix in a pixel unit 1104. A V selection means 1007 sequentially selects pixel rows. For each pixel of the selected row, in the case of ordinary incident light, the electrical charges accumulated in the photodiode are transferred to the FD unit (charge-voltage converter) through the transfer unit and converted to voltage, then outputted externally through the horizontal signal line 1116 and the output unit 1118A. If the incident light is excessive, the overflow signal charges from the photodiode are converted to a voltage by the FD unit, and is outputted externally through the horizontal signal line 1014 and the output unit 1016. At the row preceding the selection row, the V selection means 1007 resets the FD unit in advance, and ensures that the FD unit can acquire charges corresponding to the amount of the signal charge overflowing from the photodiode. The control circuit 1052 sets, in an appropriate manner, this preceding row, and controls the gain of the output units 1016 and 1118A, each of which have different amps.
However, in Japanese Patent Application Laid-Open No. 2003-101881, for each pixel column, it is necessary to have as many gain circuits as there are gain types. For this reason, noise may result from the fluctuations of the threshold values of the gain circuit, and measures to deal with this problem have been desired.